1. Field of the Invention
The present invention relates to an optical information-recording medium and a method for producing the same as well as a stamper to be used for producing a substrate of the optical information-recording medium and a method for producing the same. In particular, the present invention relates to an optical information-recording medium on which media information such as a name of manufacturer and information concerning copyright protection measures is recorded on a groove in a form of prepits and a method for producing the same as well as a stamper to be used for producing a substrate thereof and a method for producing the same.
2. Related Art
In recent years, DVD (digital versatile disk), which has a recording capacity several times that of CD (compact disk), is widely used as an information-recording medium, for example, for images and voices. In relation to DVD, those having been commercially available as products are DVD-R (write-once digital versatile disk) on which information can be recorded by the user only once and DVD-RW (rewritable digital versatile disk) on which information is rewritable. DVD-R and DVD-RW are considered to come into widespread use as large capacity information-recording media from now on.
Usually, in the case of DVD-R, predetermined information (hereinafter referred to as “media information”) including, for example, the disk manufacturer information, the information concerning copyright protection measures, and the output of laser beam to be used for recording and reproducing information on the disk is previously recorded on the disk. The media information as described above is recorded by modifying the recording layer by means of, for example, irradiation with light by using a recording apparatus at the final stage of the disk production steps.
On the other hand, Japanese Patent Application Laid-open Nos. 2000-21024, 2001-67733, and 2002-216364 disclose methods for recording the information such as media information in a form of emboss pits (hereinafter referred to as “in-groove pits”) on a groove of a substrate when the substrate of the disk is produced, without recording the information such as media information in the recording layer as described above. FIG. 21 shows a part of the substrate produced by using the method as mentioned above. FIG. 21A shows a partial magnified plan view illustrating the substrate, which schematically depicts an area (hereinafter referred to as “in-groove pit area”) in which the in-groove pits are formed. FIGS. 21B and 21C show a cross section taken along a line A′—A′ and a cross section taken along a line B′—B′ shown in FIG. 21A respectively. As shown in FIG. 21B, in the case of this substrate, the depth dp″, which ranges to a bottom surface (lowermost surface) 217a of the in-groove pit 217 on the basis of a land surface 211a of the substrate 211 formed with lands and grooves, is formed to be deeper than the depth dg″ which ranges to a bottom surface (lowermost surface) 215a of the groove 215 on the basis of the land surface 211a as well. Accordingly, when a recording layer and a reflective layer are formed on the pattern formation surface of the substrate 211 to manufacture an optical information-recording medium, the difference appears in surface height of each of the layers between the portion at which the in-groove pit 217 is formed and the groove portion at which the in-groove pit 217 is not formed. Data such as media information can be recorded on the groove by utilizing the difference in depth between the in-groove pit portion and the groove portion.
The optical information-recording medium, which uses the substrate as described above, is produced as follows. A photoresist, which is formed to have a uniform thickness on a surface of a master disk, is irradiated with a laser beam having a constant intensity to effect the exposure with a pattern corresponding to the groove. Further, the exposure is effected with a pattern corresponding to the in-groove pits by using a laser beam which is intensity-modulated to have an exposure intensity at a level higher than that of the exposure intensity of the constant intensity described above. The pattern corresponding to the groove and the pattern corresponding to the in-groove pits can be formed by performing the exposure by continuously switching the exposure intensity. Alternatively, the pattern corresponding to the in-groove pits may be formed afresh by performing the exposure after once forming the pattern corresponding to the groove by performing the exposure. Subsequently, the exposed master disk is subjected to a development treatment to form a desired photoresist pattern corresponding to the groove and the in-groove pits on the master disk. Subsequently, an etching treatment such as RIE is applied to the photoresist pattern formation surface of the master disk to form the desired pattern corresponding to the groove and the in-groove pits on the surface of the master disk. Subsequently, a stamper is manufactured by using the master disk which has the pattern formed on the surface. Further, substrates are replicated by using the stamper. A variety of layers including, for example, a recording film, are formed on the pattern formation surface of the replicated substrate. Thus, the optical information-recording medium is successfully produced.
However, when the portion corresponding to the in-groove pits is subjected to the exposure by means of the master disk-exposing method as described above, the exposure is performed for a period of time T1 corresponding to the pit length of the in-groove pit to be formed as shown in FIGS. 22A and 22B. Therefore, the length L1 in the track direction of the in-groove pit formation portion 221 formed by the exposure of the photoresist is lengthened by an amount corresponding to the diameter D of the light spot SP2 radiated onto the photoresist on the master disk. Therefore, in relation to the space 222 to be arranged between the in-groove pit formation portion 221 and another in-groove pit formation portion 221′ adjoining in the track direction, the length L2 in the track direction of the space 222 is shortened by the amount as described above. Therefore, the jitter of the signal reproduced from the in-groove pits to be read from the optical information-recording medium is increased.
Usually, as shown in FIG. 23, the following technique is known for the optical information-recording medium such as DVD-R in order to suppress the jitter of the reproduced signal obtained from the prepits. That is, notches 231a are periodically provided on parts of the groove 231. The prepits 232 are separately formed on the notch portions 231a. When a master disk, which is used to produce the optical information-recording medium as described above, is subjected to the exposure, the exposure amount, which is required to perform the exposure of a pattern corresponding to the prepits 232, can be adjusted irrelevant to the exposure amount which is required to perform the exposure of the pattern corresponding to the groove 231. Accordingly, it is possible to suppress or avoid the redundancy of the prepit length, for example, by forming the prepit to have a small size by lowering the exposure intensity when the prepit formation portion is subjected to the exposure, or by curtailing the exposure time interval for the prepit formation portion by a predetermined period of time before and after the exposure (at the exposure start end and at the exposure termination end) to make the adjustment so that the exposure time interval is shortened.
However, in the case of the optical information-recording medium in which the in-groove pits are formed, when the pit size is decreased by merely lowering the exposure intensity for the in-groove pit formation portion during the exposure of the master disk or when the exposure time for the in-groove pit formation portion is shortened by performing the curtailing before and after the exposure, then the exposure amount is insufficient at the end of the in-groove pit formation portion in the track direction. Therefore, the photoresist disposed at the portion is not subjected to the exposure to arrive at the master disk surface. For this reason, it is impossible to accurately form the desired photoresist pattern on the master disk. Further, as shown in FIG. 24, in the case of the master disk in which the surface etching is performed in accordance with such a photoresist pattern, the angle of inclination θ2 of the wall surface 242b of the in-groove pit formation portion 242 confronted in the track direction, which is measured on the basis of the bottom surface 242a of the in-groove pit formation portion 242, is decreased. The angle of inclination of the wall surface of the in-groove pit confronted in the track direction is also decreased on a substrate for the optical information-recording medium manufactured on the basis of the master disk, in the same manner as in the master disk described above. Therefore, the modulation factor of the reproduced signal obtained from the in-groove pits is lowered.
In the optical information-recording medium in which the recording layer containing an organic dye is formed on the pattern formation surface of the substrate, the difference appears in height position of the interface between the recording layer and the reflective layer due to the difference in width of the pattern formed on the substrate. The height position of the interface between the recording layer and the reflective layer relative to the wide-width groove is lower than that relative to the narrow-width groove. As a result, the difference arises between the optical path length of the laser beam at the wide-width groove portion and the optical path length of the laser beam at the narrow-width groove portion. Japanese Patent Application Laid-open No. 2001-351268 discloses a method for reproducing information such as media information by utilizing the difference in optical path length. However, the depth of the wide-width groove is the same as that of the narrow-width groove formed on the substrate which is used in this method.
However, in the optical information-recording medium described above, it is practically impossible to bring about any difference in optical path length of the laser beam to such an extent that a certain signal modulation factor, which is sufficient to reproduce information, is obtained, although the slight difference appears in height position of the interface between the recording layer and the reflective layer between the wide-width portion and the narrow-width portion of the groove. It was unsuccessful to obtain any sufficient difference in optical path length even by variously changing, for example, the dimension ratio between the wide-width portion and the narrow-width portion of the groove, the viscosity of the recording layer material, and the condition of the rotary driving of the substrate. Further, it is difficult to narrow the track pitch in the case of the optical information-recording medium on which information such as media information is recorded by utilizing the wide-width portion and the narrow-width portion of the groove as described above. Therefore, such an optical information-recording medium is especially disadvantageous to increase the recording capacity of the optical information-recording medium by narrowing the track pitch.
Japanese Patent Application Laid-open Nos. 8-129780 and 2002-237093 disclose optical information-recording media in which the film thickness of the recording layer (dye layer) formed on the substrate differs depending on the depth of the pit or the groove formed on the substrate surface. However, any one of the patent documents refers to an optical information-recording medium having in-groove pits.
An object of the present invention is to provide an optical information-recording medium having in-groove pits excellent in recording and reproduction characteristics and tracking characteristics, and a method for producing the same. Another object of the present invention is to provide a stamper to be used for producing a substrate for the optical information-recording medium and a method for producing the same.